The invention relates to an improved process for reducing the total sulfur content of Claus off-gases.
In a typical Claus process elemental sulfur is manufactured from hydrogen sulfide by partial oxidation of the hydrogen sulfide to sulfur dioxide with oxygen or an oxygen-containing gas such as air, followed by reaction of the sulfur dioxide formed with the remaining part of the hydrogen sulfide in the presence of a catalyst. This process is commonly used both at refineries and for working-up hydrogen sulfide recovered from natural gas. It is generally carried out in a plant comprising a combustion chamber followed by one or more catalyst beds having condensers arranged in between in which the reaction products are cooled and the separated liquid sulfur recovered. The various steps of the process can be represented by the following equations: ##EQU1## WHILE THE TOTAL REACTION IS REPRESENTED BY EQUATION (3): ##EQU2## For temperatures below 500.degree.C, x in the above equation has a value of 8.
In actual practice, the yield of recovered elemental sulfur is not completely quantitative resulting in a certain quantity of unreacted hydrogen sulfide and sulfur dioxide remaining in the effluent gases from the Claus process. These gases which emanate from the Claus process at temperatures typically in the 150.degree.C range are normally burned in an incinerator whereby the hydrogen sulfide is converted to sulfur dioxide which is subsequently discharged to the atmosphere through a stack. The quantity of sulfur recovered depends to a large extent on the total number of catalyst beds used in the Claus process. When three beds are usually generally about 98 percent of the sulfur can be recovered.
Because of increasingly stringent limitations on sulfur emissions to the atmosphere, and to increase sulfur yields, a considerable amount of effort has been devoted recently to reducing the sulfur content of Claus plant off-gases. Among the more desirable processes developed for this purpose are those based on the catalytic reduction of the sulfur oxides contained in the off-gases to hydrogen sulfide which is subsequently removed with the use of a solid adsorbent or liquid absorbent for hydrogen sulfide. Generally, the reduction of the off-gases is effected by mixing the gases with a hydrogen and/or carbon monoxidecontaining reducing gas in the presence of a metal catalyst at elevated temperatures, e.g., above 175.degree.C. Such catalytic hydrogenation processes are described, for example in co-assigned U.S. application Ser. No. 326,916 filed Jan. 26, 1973 and in U.S. Pat. No. 3,752,877 to Beavon.
Since a temperature differential exists between the temperature of the off-gases emanating from the Claus process, said gases generally becoming available at temperatures of approximately 150.degree.C, and the optimum temperature for catalytic reduction, preferred temperatures being above 175.degree.C, it is generally required that these off-gases be heated prior to catalytic reduction. To heat the Claus off-gases to a temperature in excess of 175.degree.C a direct heating or line burner is conventionally employed. By this is meant a burner in which gases are combusted and in which the combustion products are subsequently mixed with the gas to be heated; heating thus takes place by direct contact. Such a line burner may very suitably comprise a burner section and a mixing section. In this line burner, a carbonaceous fuel-- e.g., a hydrocarbon such as gas-oil, naphtha, coal fines, etc.-- is combusted with an oxygen-containing gas in the burner section and the hot gases are passed to the mixing section, into which the Claus off-gases, which are to be heated by the said hot gases, are also introduced. The oxygen-containing gas is introduced into the burner section in a stoichiometric proportion in relation to the fuel, so that no free oxygen is present in the hot gases which are passed to the reducing catalyst. With this method of heating Claus off-gases there is a potential hazard in connection with the consecutive addition of hydrogen-containing gas to the reduction catalyst bed or to the supply line thereto of the heated Claus off-gases. This potential hazard is due to the fact that the heated Claus off-gases leaving the direct heating burner may still contain some oxygen, for example, owing to incorrect regulation of or upsets in the oxygen and/or fuel supply. This situation creates the risk of explosions, which can occur if the oxygen-containing gases and the hydrogen-containing gases come into contact. The present invention provides a process in which the explosion hazard is eliminated.